2-stroke v 4-stroke equivalence

2-stroke v 4-stroke equivalence

Post by Jerome A. Westpha » Sun, 05 Aug 2001 01:06:15



When considering 2-stroke versus 4-stroke engines, equivalences seem to be
based on a torque comparison.  For instance, an .80 ci 4-stroke may be
recommended as an equivalent to a  .60 ci 2-stroke.

Since power (nominally the rate of application of torque) is what enables
the plane to fly, why aren't the engine comparisons based on horsepower?

I'm thinking of putting a 4-stroke in a Goldberg Tiger 60, and they
recommend a .45 to .60 2 cycle or a .65 to .80 4-cycle and this seems to be
consistant with engine performance charts I've seen.  It looks to me like a
.7 4-stroke may be rated at 1.1 HP whereas a .46 2-stoke may be rated at
about 1.6 HP.  A .9 4-stroke may put out about 1.6 HP as opposed to 1.6 HP
for a .6 2-stroke.

What is it that I'm missing here?

--
Jerome A. Westphal

 
 
 

2-stroke v 4-stroke equivalence

Post by LearJet35 » Sun, 05 Aug 2001 01:36:45


general rule of thumb....

2 stroke =  4 stroke
  .25     -      .30
  .40la   -     .56
  .40fx   -     .65
  .46fx   -     .72
  .61fx   -     .91
  .91fx   -    1.20
1.08      -    1.50
1.35      -    1.80

If you are considering the Saito 80... spend the extra few bucks and get the
Saito 91... lighter and more power.....  run a 14x6 on the 91 for best
pulling power....



Quote:
> When considering 2-stroke versus 4-stroke engines, equivalences seem to be
> based on a torque comparison.  For instance, an .80 ci 4-stroke may be
> recommended as an equivalent to a  .60 ci 2-stroke.

> Since power (nominally the rate of application of torque) is what enables
> the plane to fly, why aren't the engine comparisons based on horsepower?

> I'm thinking of putting a 4-stroke in a Goldberg Tiger 60, and they
> recommend a .45 to .60 2 cycle or a .65 to .80 4-cycle and this seems to
be
> consistant with engine performance charts I've seen.  It looks to me like
a
> .7 4-stroke may be rated at 1.1 HP whereas a .46 2-stoke may be rated at
> about 1.6 HP.  A .9 4-stroke may put out about 1.6 HP as opposed to 1.6 HP
> for a .6 2-stroke.

> What is it that I'm missing here?

> --
> Jerome A. Westphal


 
 
 

2-stroke v 4-stroke equivalence

Post by Jim Arche » Sun, 05 Aug 2001 01:36:14


What you are missing is :-
1. The 'X' bhp at 'Y' rpm.
2. Real world figures.

To explain - most manufacturers claimed bhp figures for their engines are
never actually achievable, to even get close to them would require using a
prop so small that it would be totally inapropriate to 'normal' model
flying.

Taking your figure of 1.6 bhp for a .46 2 stroke, this _may_ be achievable,
but would probably involve (e.g.) turning a 9 x 6 at 18000 rpm.
On the other hand, the 4 stroke will reach it's peak bhp at much lower rpm
figures, allowing you to use a more sensibly sized prop. A (larger) 4 stroke
might (again e.g.) turn a 13 x 6 at around 9500 rpm, giving you more static
thrust, and a sound that you and your neighbours might be able to live
with...........

--
Best regards

Jim Archer
Norwich. UK



Quote:
> When considering 2-stroke versus 4-stroke engines, equivalences seem to be
> based on a torque comparison.  For instance, an .80 ci 4-stroke may be
> recommended as an equivalent to a  .60 ci 2-stroke.

> Since power (nominally the rate of application of torque) is what enables
> the plane to fly, why aren't the engine comparisons based on horsepower?

> I'm thinking of putting a 4-stroke in a Goldberg Tiger 60, and they
> recommend a .45 to .60 2 cycle or a .65 to .80 4-cycle and this seems to
be
> consistant with engine performance charts I've seen.  It looks to me like
a
> .7 4-stroke may be rated at 1.1 HP whereas a .46 2-stoke may be rated at
> about 1.6 HP.  A .9 4-stroke may put out about 1.6 HP as opposed to 1.6 HP
> for a .6 2-stroke.

> What is it that I'm missing here?

> --
> Jerome A. Westphal

 
 
 

2-stroke v 4-stroke equivalence

Post by Ian Maclaughli » Sun, 05 Aug 2001 02:48:35



Quote:
>When considering 2-stroke versus 4-stroke engines, equivalences seem to be
>based on a torque comparison.  For instance, an .80 ci 4-stroke may be
>recommended as an equivalent to a  .60 ci 2-stroke.

>Since power (nominally the rate of application of torque) is what enables
>the plane to fly, why aren't the engine comparisons based on horsepower?

>I'm thinking of putting a 4-stroke in a Goldberg Tiger 60, and they
>recommend a .45 to .60 2 cycle or a .65 to .80 4-cycle and this seems to be
>consistant with engine performance charts I've seen.  It looks to me like a
>.7 4-stroke may be rated at 1.1 HP whereas a .46 2-stoke may be rated at
>about 1.6 HP.  A .9 4-stroke may put out about 1.6 HP as opposed to 1.6 HP
>for a .6 2-stroke.

>What is it that I'm missing here?

Jerome-
Good analysis, but IMHO the first assumption is probably incorrect.  I
think most modelers consider equivalence of the engine types to mean
what produces equivalent performance, and as you observed, power is
what enables the plane to fly.  The forte of 4-stroke engines is the
ability to produce good power in lower rpm ranges that are usable in
model aircraft.  To produce the same power at lower rpm, obviously,
according to the relationship between power and torque you cited, they
must produce more torque than their 2-stroke 'equals.'  
Your Tiger 60 is a good platform for an example.  
With a Saito .65 or .72 -stroke and APC 11X8, rpm will be 10,500 -
11,000.  Thrust is in the range of 4.7 - 5 lb, and speed potential
about 80 mph.  This is at a power level of .95 - 1.1 bhp, which these
engines can produce comfortably.
With a .46 2-stroke that is hot (capable of producing actual power
comparable to the 1.1 bhp produced by the Saito .72 - the 1.6 hp
figure is ad man BS), a 9X7 prop will turn at 15,000+, thrust is in
the same ballpark at about 4.5 lb, and speed potential is near 100
mph.
Overall the performance is about same.  Though speed potential is
higher for the .46 2-stroke in this example, it will never be reached
using either engine on the Tiger 60, which will likely top out at
about 60 mph.  The most significant difference to me is that at WOT
the Saito is running at a comfortable rpm and the noise level produced

sites.  The .46 is screaming at near racing rpms, and prop generated
noise level is twice at great, 96+ dBA, which is not acceptable at
many sites.
BTW, a .46 that is actually capable of 1.6 bhp will turn that 9X7 APC

smallest recommended prop is a 10.5X6, so I'll use that.  At 1.6 bhp,
the .46 FX should turn that prop at 14,500.  Wanna bet it will?

Ian
San Diego

 
 
 

2-stroke v 4-stroke equivalence

Post by Bob S » Sun, 05 Aug 2001 05:32:08




Quote:

> What is it that I'm missing here?

One word:  "torque."

An engine's torque output at realistic commonly-used RPM settings tells you
a lot more about what it can do than HP numbers!

Good flying,
Bob Scott

 
 
 

2-stroke v 4-stroke equivalence

Post by D.G » Sun, 05 Aug 2001 06:39:44


...

Quote:
> > What is it that I'm missing here?

> One word:  "torque."

> An engine's torque output at realistic commonly-used RPM settings tells you
> a lot more about what it can do than HP numbers!

Moreover, power is pretty much torque*rpm. Go figure. It really depends
on the prop you want to use. Large props with high pitch need a lot of
torque to turn at reasonable speeds. And usually, larger and slower
turning props are more efficient than high rpm suckers due to various
aerodynamic effects. And of course a 4-stroke does make a nicer sound.

 - D.G

--
----
A-10 Project
http://geocities.com/powersof2000/main/main.html

 
 
 

2-stroke v 4-stroke equivalence

Post by Brian D. Felic » Sun, 05 Aug 2001 06:54:57


Jerome,

    Well, power requirements for an airplane, or anything for that matter, are
based on........ power :-) Torque is only a means of generating power and is a
useless quantity given by itself.

    I believe you're confusing torque, power, and [manufacturer's power output
claims]. It would take quite a hot .46- 2 cycle, with full length tuned pipe,
turning very fast, to achieve anything even approaching 1.6 Hp, while a .91 4
cycle can easily achieve or even exceed this level. Further, there are
tremendous differences in power output among engines of the same nominal size.
For example, both an OS FP .60 and a Jett sport .60 have the same mechanical
displacement but no two engines (of the same size) could possibly be more
different regarding power output- the Jett will probably produce three times the
FP's best possible power level.

    Because the manufacturers have to have some method of sizing their planes,
and associating them with some reasonable engine selection, they use
displacement. It's certainly not ideal nor is it fool proof but it's a
reasonable 'handle' to place on their products. Perhaps a better method would be
to state the best engine weight, or range of engine weight for a given plane and
allow the customer to choose how much power to use given those limits.

    As a general but sometimes lousy 'rule of thumb', you can use the conversion
of 1 1/2 times bigger 4 cycle instead of a 2 cycle. So a nominal .60 sized 2
cycle would be equivalent to a .90 size 4 cycle. Unfortunately, the range and
type of performance sometimes differs between these engine types so a direct
comparison is often impossible. The best recommendation I can give for choosing
an engine is to go by the all-up weight of the aircraft and the performance you
want from it.

Brian

http://users.ids.net/~bdfelice/

"You can always tell an Engineer....

but you can't tell him much."

Quote:

> When considering 2-stroke versus 4-stroke engines, equivalences seem to be
> based on a torque comparison.  For instance, an .80 ci 4-stroke may be
> recommended as an equivalent to a  .60 ci 2-stroke.

> Since power (nominally the rate of application of torque) is what enables
> the plane to fly, why aren't the engine comparisons based on horsepower?

> I'm thinking of putting a 4-stroke in a Goldberg Tiger 60, and they
> recommend a .45 to .60 2 cycle or a .65 to .80 4-cycle and this seems to be
> consistant with engine performance charts I've seen.  It looks to me like a
> .7 4-stroke may be rated at 1.1 HP whereas a .46 2-stoke may be rated at
> about 1.6 HP.  A .9 4-stroke may put out about 1.6 HP as opposed to 1.6 HP
> for a .6 2-stroke.

> What is it that I'm missing here?

> --
> Jerome A. Westphal

 
 
 

2-stroke v 4-stroke equivalence

Post by emc.. » Sun, 05 Aug 2001 13:37:16


Typically choose a 4 stroke with 50% more displacement than a 2
stroke.

If your running a .60 2 stroke use a .91 4 stroke. A .91 is the
minimum 4 stroke I would use on a .60 size Tiger 2.

Comparing 2 strokes to 4 strokes is like comparing gas automotive
engines to diesel automotive engines.

A car with a gas engine is faster, but the diesel produce a lot more
torque at a lot less rpms. A truck with a 300 hp gas engine would tow
a heavy trailer fine, but switch to a 150 hp diesel engine and you
will accelerate faster and burn 50% less fuel.

The torque (displacement) is the missing link. You would never replace
a 300 hp gas engine with a 300 hp diesel engine. The same is partly
true for switching from a 2 stroke to a 4 stroke engine.

A 4 stroke is going to rev 3,000 to 4,000 rpms slower than the 2
stroke it is replacing. Therefore you need more torque (not more hp)
to swing a 2" or 3" larger prop to over come the lost rpm.

Or at least this is the way, I make sense of it.

Also the .91 4 stroke is not going to burn much more, if any more fuel
than the .46 2 stroke.

On Fri, 3 Aug 2001 09:06:15 -0700, "Jerome A. Westphal"

Quote:

>When considering 2-stroke versus 4-stroke engines, equivalences seem to be
>based on a torque comparison.  For instance, an .80 ci 4-stroke may be
>recommended as an equivalent to a  .60 ci 2-stroke.

>Since power (nominally the rate of application of torque) is what enables
>the plane to fly, why aren't the engine comparisons based on horsepower?

>I'm thinking of putting a 4-stroke in a Goldberg Tiger 60, and they
>recommend a .45 to .60 2 cycle or a .65 to .80 4-cycle and this seems to be
>consistant with engine performance charts I've seen.  It looks to me like a
>.7 4-stroke may be rated at 1.1 HP whereas a .46 2-stoke may be rated at
>about 1.6 HP.  A .9 4-stroke may put out about 1.6 HP as opposed to 1.6 HP
>for a .6 2-stroke.

>What is it that I'm missing here?

>--
>Jerome A. Westphal

 
 
 

2-stroke v 4-stroke equivalence

Post by Chris Hind » Mon, 06 Aug 2001 06:13:40


I like the analagy actually.  Remarkably apt.  I drive a Diesel and a Gas
car.  Both have the same power to weight ratio but the Diesel is far more
driveable.  I liken this to the two stroke/four stroke debate.  The gas car
is like a 2-stroke, it slows on the upline (hills), the Diesel is like a
4-stroke, it just keeps chugging.

Chris

Quote:

> Typically choose a 4 stroke with 50% more displacement than a 2
> stroke.

> If your running a .60 2 stroke use a .91 4 stroke. A .91 is the
> minimum 4 stroke I would use on a .60 size Tiger 2.

> Comparing 2 strokes to 4 strokes is like comparing gas automotive
> engines to diesel automotive engines.

> A car with a gas engine is faster, but the diesel produce a lot more
> torque at a lot less rpms. A truck with a 300 hp gas engine would tow
> a heavy trailer fine, but switch to a 150 hp diesel engine and you
> will accelerate faster and burn 50% less fuel.

> The torque (displacement) is the missing link. You would never replace
> a 300 hp gas engine with a 300 hp diesel engine. The same is partly
> true for switching from a 2 stroke to a 4 stroke engine.

> A 4 stroke is going to rev 3,000 to 4,000 rpms slower than the 2
> stroke it is replacing. Therefore you need more torque (not more hp)
> to swing a 2" or 3" larger prop to over come the lost rpm.

> Or at least this is the way, I make sense of it.

> Also the .91 4 stroke is not going to burn much more, if any more fuel
> than the .46 2 stroke.

> On Fri, 3 Aug 2001 09:06:15 -0700, "Jerome A. Westphal"

> >When considering 2-stroke versus 4-stroke engines, equivalences seem to
be
> >based on a torque comparison.  For instance, an .80 ci 4-stroke may be
> >recommended as an equivalent to a  .60 ci 2-stroke.

> >Since power (nominally the rate of application of torque) is what enables
> >the plane to fly, why aren't the engine comparisons based on horsepower?

> >I'm thinking of putting a 4-stroke in a Goldberg Tiger 60, and they
> >recommend a .45 to .60 2 cycle or a .65 to .80 4-cycle and this seems to
be
> >consistant with engine performance charts I've seen.  It looks to me like
a
> >.7 4-stroke may be rated at 1.1 HP whereas a .46 2-stoke may be rated at
> >about 1.6 HP.  A .9 4-stroke may put out about 1.6 HP as opposed to 1.6
HP
> >for a .6 2-stroke.

> >What is it that I'm missing here?

> >--
> >Jerome A. Westphal

 
 
 

2-stroke v 4-stroke equivalence

Post by Chris Stewar » Tue, 07 Aug 2001 22:32:30


In my own experience a .60 2 stroke would not be equivalent to a .90
four stroke.
I have a .68 MDS and a .70 OS4 they both use a 13x6 prop as my favorite
they both turn about the same RPM on the ground the MDS gives better
performance on the same airplane. I tried using a smaller prop on the
MDS but did not see a difference in power. To me the smaller pitch props
(higher rpm)just don't fly as smooth. I fly aerobatics type planes so I
am not trying to go real fast. Also I don't see any difference in fuel
consumption. I was concerned about that when I built my TopCap. The
plans call for a 6 oz tank in the wing. Since I was going to use that
.68MDS instead of a .46 I decided to add another tank on the other side
of the wing for a total of 12 oz. Now I never run out of fuel because I
get tired of flying after 15 or 20 minutes. As far as sound goes, the
MDS with that big prop is not much different than the 4 stroke.

As far as the original post is concerned:
  Disregard the manufacturers claims about anything.
I would say an .60 two stoke will fly like a .70 4 stoke except for YS
four stokes which are equal or better than a 2s of the same
displacement. Of course as mentioned before different engines of the
same displacement are not the same. I have not compared an OS 60 2s so I
may find something different if I did.
--

:-) Chris Stewart
Check out my scratch built 27% Edge540T project at:
http://www.FoundCollection.com/***.COM/540x2project/cmsframeset.htm

Quote:

> Typically choose a 4 stroke with 50% more displacement than a 2
> stroke.

> If your running a .60 2 stroke use a .91 4 stroke. A .91 is the
> minimum 4 stroke I would use on a .60 size Tiger 2.

> Comparing 2 strokes to 4 strokes is like comparing gas automotive
> engines to diesel automotive engines.

> A car with a gas engine is faster, but the diesel produce a lot more
> torque at a lot less rpms. A truck with a 300 hp gas engine would tow
> a heavy trailer fine, but switch to a 150 hp diesel engine and you
> will accelerate faster and burn 50% less fuel.

> The torque (displacement) is the missing link. You would never replace
> a 300 hp gas engine with a 300 hp diesel engine. The same is partly
> true for switching from a 2 stroke to a 4 stroke engine.

> A 4 stroke is going to rev 3,000 to 4,000 rpms slower than the 2
> stroke it is replacing. Therefore you need more torque (not more hp)
> to swing a 2" or 3" larger prop to over come the lost rpm.

> Or at least this is the way, I make sense of it.

> Also the .91 4 stroke is not going to burn much more, if any more fuel
> than the .46 2 stroke.

> On Fri, 3 Aug 2001 09:06:15 -0700, "Jerome A. Westphal"


 
 
 

2-stroke v 4-stroke equivalence

Post by Paul McIntos » Wed, 08 Aug 2001 02:01:52


Your MDS must be the biggest dog around, or your OS is on steroids!  There
are plenty of .46-.50 two strokes that will outrun the OS .70.

--
Paul McIntosh
Desert Sky Model Aviation
http://www.FoundCollection.com/

Quote:
> In my own experience a .60 2 stroke would not be equivalent to a .90
> four stroke.
> I have a .68 MDS and a .70 OS4 they both use a 13x6 prop as my favorite
> they both turn about the same RPM on the ground the MDS gives better
> performance on the same airplane. I tried using a smaller prop on the
> MDS but did not see a difference in power. To me the smaller pitch props
> (higher rpm)just don't fly as smooth. I fly aerobatics type planes so I
> am not trying to go real fast. Also I don't see any difference in fuel
> consumption. I was concerned about that when I built my TopCap. The
> plans call for a 6 oz tank in the wing. Since I was going to use that
> .68MDS instead of a .46 I decided to add another tank on the other side
> of the wing for a total of 12 oz. Now I never run out of fuel because I
> get tired of flying after 15 or 20 minutes. As far as sound goes, the
> MDS with that big prop is not much different than the 4 stroke.

> As far as the original post is concerned:
>   Disregard the manufacturers claims about anything.
> I would say an .60 two stoke will fly like a .70 4 stoke except for YS
> four stokes which are equal or better than a 2s of the same
> displacement. Of course as mentioned before different engines of the
> same displacement are not the same. I have not compared an OS 60 2s so I
> may find something different if I did.
> --

> :-) Chris Stewart
> Check out my scratch built 27% Edge540T project at:
> http://www.FoundCollection.com/***.COM/540x2project/cmsframeset.htm


> > Typically choose a 4 stroke with 50% more displacement than a 2
> > stroke.

> > If your running a .60 2 stroke use a .91 4 stroke. A .91 is the
> > minimum 4 stroke I would use on a .60 size Tiger 2.

> > Comparing 2 strokes to 4 strokes is like comparing gas automotive
> > engines to diesel automotive engines.

> > A car with a gas engine is faster, but the diesel produce a lot more
> > torque at a lot less rpms. A truck with a 300 hp gas engine would tow
> > a heavy trailer fine, but switch to a 150 hp diesel engine and you
> > will accelerate faster and burn 50% less fuel.

> > The torque (displacement) is the missing link. You would never replace
> > a 300 hp gas engine with a 300 hp diesel engine. The same is partly
> > true for switching from a 2 stroke to a 4 stroke engine.

> > A 4 stroke is going to rev 3,000 to 4,000 rpms slower than the 2
> > stroke it is replacing. Therefore you need more torque (not more hp)
> > to swing a 2" or 3" larger prop to over come the lost rpm.

> > Or at least this is the way, I make sense of it.

> > Also the .91 4 stroke is not going to burn much more, if any more fuel
> > than the .46 2 stroke.

> > On Fri, 3 Aug 2001 09:06:15 -0700, "Jerome A. Westphal"


 
 
 

2-stroke v 4-stroke equivalence

Post by elb.. » Wed, 08 Aug 2001 02:16:30


I think you missed the point. I was refering to switching from a 2
stroke to an equivalent size 4 stroke.

Consider a plane such as a 4 Star .60 that requires a absolute minimum
of a 2 cycle .60. I very much doubt that it would be able to fly with
a 4 stroke .70. Prehaps not even get off the ground.

I have seen a 4 Star 60's fly with MDS .68 and it flies great. I have
also flown 4 Star 60's with 4 stroke OS .91's, Saito .91's, Magnum
91's, and TT .91's. I find the .91 4 stroke to be the absolute
smallest 4 stroke I would put on this plane. The OS .70's are
wonderful great running engines (many praises to OS) but its not
enough power for this plane.

You appear to be using both the MDS .68 and OS .70 on a .40 size
plane. YES, on a .40 size plane you could get by day after day running
a 13x6 on both engines in the 9,500 rpms range. But put the MDS .68 on
a 4 Star 60 and your engine will overheat trying to pull a 13x6 with
the increased load of a larger/heavier airframe. To get the power
needed to fly, you would have to run the engine in the 11,000+ rpm
range, which means a smaller prop.

I stick firmly by my claim, if you have a airplane that requires the
absolute minimum of a .60 two cycle you had better seriously consider
at least a 50% larger .91 4 stroke.

On a already seriously overpowered .40 size plane. NO, your not going
to see a big difference in a .60 size two cycle or a .70 four stroke.
Overpowered is overpowered. Underpowered will leave you sweeping
debris off the runway.

Put a OS .40 four stroke on your .40 size TopCap and I doubt it will
get off the ground.

The opposite is also true, if you have a plane that is currently
flying with a .70 4 stroke it should also fly on a 33% smaller .46
size 2 cycle engine.

.60 -> .91 = 50% increase
.91 -> .60 = 33% decrease

On Mon, 06 Aug 2001 08:32:30 -0500, Chris Stewart

Quote:

>In my own experience a .60 2 stroke would not be equivalent to a .90
>four stroke.
>I have a .68 MDS and a .70 OS4 they both use a 13x6 prop as my favorite
>they both turn about the same RPM on the ground the MDS gives better
>performance on the same airplane. I tried using a smaller prop on the
>MDS but did not see a difference in power. To me the smaller pitch props
>(higher rpm)just don't fly as smooth. I fly aerobatics type planes so I
>am not trying to go real fast. Also I don't see any difference in fuel
>consumption. I was concerned about that when I built my TopCap. The
>plans call for a 6 oz tank in the wing. Since I was going to use that
>.68MDS instead of a .46 I decided to add another tank on the other side
>of the wing for a total of 12 oz. Now I never run out of fuel because I
>get tired of flying after 15 or 20 minutes. As far as sound goes, the
>MDS with that big prop is not much different than the 4 stroke.

>As far as the original post is concerned:
>  Disregard the manufacturers claims about anything.
>I would say an .60 two stoke will fly like a .70 4 stoke except for YS
>four stokes which are equal or better than a 2s of the same
>displacement. Of course as mentioned before different engines of the
>same displacement are not the same. I have not compared an OS 60 2s so I
>may find something different if I did.
>--

>:-) Chris Stewart
>Check out my scratch built 27% Edge540T project at:
>http://www.FoundCollection.com/***.COM/540x2project/cmsframeset.htm


>> Typically choose a 4 stroke with 50% more displacement than a 2
>> stroke.

>> If your running a .60 2 stroke use a .91 4 stroke. A .91 is the
>> minimum 4 stroke I would use on a .60 size Tiger 2.

>> Comparing 2 strokes to 4 strokes is like comparing gas automotive
>> engines to diesel automotive engines.

>> A car with a gas engine is faster, but the diesel produce a lot more
>> torque at a lot less rpms. A truck with a 300 hp gas engine would tow
>> a heavy trailer fine, but switch to a 150 hp diesel engine and you
>> will accelerate faster and burn 50% less fuel.

>> The torque (displacement) is the missing link. You would never replace
>> a 300 hp gas engine with a 300 hp diesel engine. The same is partly
>> true for switching from a 2 stroke to a 4 stroke engine.

>> A 4 stroke is going to rev 3,000 to 4,000 rpms slower than the 2
>> stroke it is replacing. Therefore you need more torque (not more hp)
>> to swing a 2" or 3" larger prop to over come the lost rpm.

>> Or at least this is the way, I make sense of it.

>> Also the .91 4 stroke is not going to burn much more, if any more fuel
>> than the .46 2 stroke.

>> On Fri, 3 Aug 2001 09:06:15 -0700, "Jerome A. Westphal"


 
 
 

2-stroke v 4-stroke equivalence

Post by elb.. » Wed, 08 Aug 2001 04:35:10


I would consider them equal more or less.

.46 -> .70 = 50% larger displacement more or less. 50% is just a ball
park figure.

A 4 stroke .48, .50, .52, .56 are substitutes for a weak to strong 2
cycle .40

A 4 stroke .60, .65, 70, 72 are substitutes for weak to strong .46 two
cycles engines.

There again ball park figures.

If it is a plane designed for speed and you want to go fast the .46
would be the optimum choice.

If it is a plane that handles well at slower speed and does not
require lost of speed to fly well I would also use the .70

4 strokes normally swing a 1" to 2" larger prop than the 2 cycle
engine their replacing. Which means they are moving up to 20% move
volume of air, however at a slower velocity.

The extra volume of air under the wing means a quick take-off, slower
stall speed, and other advantages.

A 2 cycle does not have these advantages, but the .46 should turn
3,000 to 4,000 and prehaps 5,000 rpms faster than the .70 four stroke.

The advantage to the 2 cycle is speed, prehaps nearly 50% faster.

An OS .70 with 13x6 prop, I'm guessing 9,500 rpms. An OS .46 with 11x6
prop, again guessing 12,000 rpms or a 10x6 prop maybe 13,500 rpms.

On Mon, 06 Aug 2001 14:53:53 -0400, Joe Helmick

Quote:

>This raises a question:  Do you consider an OS FS 70 Surpass (not the
>Surpass II) to be equivalent to an OS 46 FX?  What does everyone else
>think about this?

>I'm scratchbuilding from plans which call for either engine, but I was
>planning on installing the FS 70.  It's my first 4-stroke and when I was
>running it in, I was surprised by the SERIOUS breeze it blew up,
>compared to my 46.

>Opinions, anyone?

>Joe


>>  The opposite is also true, if you have a plane that is currently
>>  flying with a .70 4 stroke it should also fly on a 33% smaller .46
>>  size 2 cycle engine.

 
 
 

2-stroke v 4-stroke equivalence

Post by Mitc » Wed, 08 Aug 2001 05:12:07


Brian,
Care to voice your opinion as to "why" four strokes model engines are
torquier than two strokes?  Is it just by design?  I know single cylinder
thumper motorcycles will, normally, outpull big two strokes.  But if a two
stroke is designed for torque, through port timing and flywheel weight, they
come awfully close to big thumpers in grunt power.  Think model airplane two
stroke engines are designed and timed more for rpm?  Or is it something
about four strokes that just naturally makes them develop more torque?



Quote:
> Jerome,

>     As a general but sometimes lousy 'rule of thumb', you can use the
conversion
> of 1 1/2 times bigger 4 cycle instead of a 2 cycle. So a nominal .60 sized
2
> cycle would be equivalent to a .90 size 4 cycle. Unfortunately, the range
and
> type of performance sometimes differs between these engine types so a
direct
> comparison is often impossible. The best recommendation I can give for
choosing
> an engine is to go by the all-up weight of the aircraft and the
performance you
> want from it.

 
 
 

2-stroke v 4-stroke equivalence

Post by elb.. » Wed, 08 Aug 2001 05:34:13


A 2 cycle engine has fuel and exhaust ports in the cylinder walls. The
piston fires, the gases expand, the piston comes down, then BOOM,
right during the power phase the piston moves pass the exhaust port
and the expanding gas is blown out the muffler. Very inefficent.

A 4 stroke has valves. During the power stroke the fuel ignites and
the cylinder stay completely sealed during the stroke. The gases
expand forcing the piston all the way to the bottom of the cylinder.
Its the increased longer STROKE that creates the torque. The down
force on the piston is sustained for a longer period of time.

The same as a long stroked truck engine creates more torque than a
short stroked car engine.

If you read old magazine this is also why the OS .61 long stroke 2
cycle was praised. It had a longer stroke than a typical 2 cycle, it
would swing a longer prop.

Bore increases hp, that is why the newer 4 strokes have more hp. The
manufactures have started producing large bore/shorter stroked engine.
Shorter stroke more rpms, more hp, but less torque. I want the long
stroke 4 strokes back.

Quote:

>Brian,
>Care to voice your opinion as to "why" four strokes model engines are
>torquier than two strokes?  Is it just by design?  I know single cylinder
>thumper motorcycles will, normally, outpull big two strokes.  But if a two
>stroke is designed for torque, through port timing and flywheel weight, they
>come awfully close to big thumpers in grunt power.  Think model airplane two
>stroke engines are designed and timed more for rpm?  Or is it something
>about four strokes that just naturally makes them develop more torque?



>> Jerome,

>>     As a general but sometimes lousy 'rule of thumb', you can use the
>conversion
>> of 1 1/2 times bigger 4 cycle instead of a 2 cycle. So a nominal .60 sized
>2
>> cycle would be equivalent to a .90 size 4 cycle. Unfortunately, the range
>and
>> type of performance sometimes differs between these engine types so a
>direct
>> comparison is often impossible. The best recommendation I can give for
>choosing
>> an engine is to go by the all-up weight of the aircraft and the
>performance you
>> want from it.